The long term goal of this project is to understand intracellular reactive signaling pathways involved in cardiac hypertrophy. In response to various forms of heart disease the myocardium undergoes hypertrophic growth to compensate for a loss in cardiac output. While this response is initially beneficial, it eventually leads to pathology and heart failure. Despite years of investigator, the molecular pathways that result in cardiac hypertrophy remain largely unknown. The transcription GATA4 has been shown to play an important role in regulating the hypertrophic response at the transcriptional level. To investigate the molecular mechanisms whereby GATA4 regulates the hypertrophic responses, a yeast 2-hybrid screen was performed which identified the transcription factor NF-AT3 (nuclear factor of activated T-cells). Studies in T-cells have demonstrated that NF-AT factors are sequestered in the cytoplasm until activated by the phosphatase calcineurin in response to stress or stimuli that increase intracellular calcium. Our recent results demonstrate that the myocardium also utilizes this stress-response signaling pathway. Transgenic mice expressing a constitutively active form of calcineurin or a constitutively nuclear NF-AT3 protein in the heart develop substantial hypertrophy resulting in heart failure. Furthermore, inhibition of endogenous calcineurin activity with the drug cyclosporin A prevents angiotensin II and adrenergic-mediated hypertrophy of cultured neonatal cardiomyocytes. These data suggest a model whereby calcineurin regulates cardiac hypertrophy by effecting the transcription factor NF- AT3 and its ability to activated hypertrophic response genes. However, the physiologic relevance of this novel signaling pathway is presently uncertain. To this end, the goals of the proposed study are: 1) to define the mechanisms of calcineurin hypertrophic signaling in cultured neonatal cardiomyocytes. 2) To use transgenic models of intrinsic heart disease to determine the role of calcineurin signaling in genetically mediated myopathic responses. 3) To determine the role of calcineurin signaling in models of extrinsic reactive hypertrophy. 4) To determine if calcineurin is activated in human heart failure. Transgenic and pathophysiologic animal models as well as in vitro culture systems are proposed to test the hypothesis that calcineurin/NF-AT act as a parallel regulatory pathway for cardiac reactive responses.